The problem with getting the flyers first is you get absolutely no benefit until all 5 have them so you are in crappy gear and have crappy mounts until you get the full 25k gold. You can start upgrading gear right away and see the benefit instantly.

My DK team still has regular flyers and it's rare situations I find myself wishing they had the epics. You use them to get to raids and do dailies, I don't even farm as doing dailies makes money faster so there isn't really much benefit to them.